Ending an Activation is stated pretty clearly. "Once a character has spent all of its AD pool his or her activation is over." This means any abilities a character may have that do not require AD cannot be used after the characters last AD is expended. That is how I will play it, but it feels arbitrary and counter-intuitive. Cave Bear Frenzy ability, or Writ of Justice could no longer be used once the last AD is used, for example.

Hmm, I think you have a reasonable point. I don't think I'd have an issue with a player spending his action points, then activating the cave bear frenzy ability. I'll pitch the idea to Sean and see what he says.